In the field of wireless communication, for many applications, such as within a cellular radio infrastructure, radio frequency (RF) power amplifiers (PAs) are continuously being required to support increasingly greater instantaneous bandwidths. The instantaneous bandwidth, or video bandwidth as it is often referred to, is the maximum signal bandwidth that an amplifier is able to properly amplify a communication signal. In particular, the progressive generations of wireless communication standards, such as the Global System for Mobile Communication (GSM) standard and the more recent 3rd Generation Partnership Project (3GPP) standard, have successively required increased bit rates. Accordingly, each generation has required an increased instantaneous bandwidth, and the future generations being considered are requiring that these bit rates are extended even further.
FIG. 1 illustrates schematically a typical example of a RF PA circuit 100 comprising a first power amplifier block 110 and a second power amplifier block 120 operably coupled in parallel between inputs 102, 104 and outputs 170, 175 respectively, for example as may be used within RF integrated circuits (RFICs). Each power amplifier block 110, 120 comprises an input matching circuit 130, 140 operably coupling the respective RF input signal 102, 104 to a respective RF PA transistor 112, 122. The output of a drain port of each RF PA transistor 112, 122 is operably coupled to an output matching circuit 150, 160. The RF PA circuit 100 may be configured to operate as a push-pull (differential) amplifier, whereby inputs 102, 104 comprise differential inputs and the outputs 170, 175 comprise differential outputs. Equally, the RF PA circuit may be configured as a single ended amplifier circuit by operably coupling together the two inputs 102, 104 and the two outputs 170, 175 respectively.
A problem with this known RF PA design, in either the differential configuration or the single ended configuration, is that the instantaneous bandwidth is limited by capacitors 155, 165 within the output matching circuits 150, 160, typically to an instantaneous bandwidth of no more than around 30 MHz. Capacitors 155, 165 provide DC blocking for the respective shunt inductances 157, 167, whilst providing a short circuit for RF frequency signals such that the parallel reactance at such RF frequencies on the drain port of the respective transistor 112, 122 comprises an inductance. Typically the capacitors 155, 165 comprise a capacitance value in the region of 200 pF. Lower value capacitances are typically unsuitable since they cannot provide a good enough radio frequency (RF) short. However, existing semiconductor technologies prevent higher value capacitances from being used because of restrictions in capacitance density. This limitation to the value of capacitors 155, 165 limits the instantaneous bandwidth achievable, thus restricting the suitability of such an amplifier circuit for present and future RF PA drain modulation systems. In addition, the lossy nature of the capacitors 155, 165 affects the peak efficiency of the amplifier circuit 100 (by a few percent). Furthermore, the single ended configuration is sensitive to ground connection inductance, which affects unit-to-unit (e.g. RF integrated circuit) RF performance dispersion.